EAP data is first encapsulated in EAPOL frames between the Supplicant and Authenticator, then re-encapsulated between the Authenticator and the Authentication server using RADIUS or Diameter.

802.1X authentication involves three parties: a supplicant, an authenticator, and an authentication server. The supplicant is a client device (such as a laptop) that wishes to attach to the LAN/WLAN. The term 'supplicant' is also used interchangeably to refer to the software running on the client that provides credentials to the authenticator. The authenticator is a network device, such as an Ethernet switch or wireless access point; and the authentication server is typically a host running software supporting the RADIUS and EAP protocols. In some cases, the authentication server software may be running on the authenticator hardware.

The authenticator acts like a security guard to a protected network. The supplicant (i.e., client device) is not allowed access through the authenticator to the protected side of the network until the supplicant’s identity has been validated and authorized. An analogy to this is providing a valid visa at the airport's arrival immigration before being allowed to enter the country. With 802.1X port-based authentication, the supplicant provides credentials, such as user name/password or digital certificate, to the authenticator, and the authenticator forwards the credentials to the authentication server for verification. If the authentication server determines the credentials are valid, the supplicant (client device) is allowed to access resources located on the protected side of the network.[7]

802.1X-2001 defines two logical port entities for an authenticated port—the "controlled port" and the "uncontrolled port". The controlled port is manipulated by the 802.1X PAE (Port Access Entity) to allow (in the authorized state) or prevent (in the unauthorized state) network traffic ingressing and egressing to/from the controlled port. The uncontrolled port is used by the 802.1X PAE to transmit and receive EAPOL frames.

802.1X-2004 defines the equivalent port entities for the supplicant; so a supplicant implementing 802.1X-2004 may prevent higher level protocols being used if it is not content that authentication has successfully completed. This is particularly useful when an EAP method providing mutual authentication is used, as the supplicant can prevent data leakage when connected to an unauthorized network.

Initialization On detection of a new supplicant, the port on the switch (authenticator) is enabled and set to the "unauthorized" state. In this state, only 802.1X traffic is allowed; other traffic, such as the Internet Protocol (and with that TCP and UDP), is dropped.

Initiation To initiate authentication the authenticator will periodically transmit EAP-Request Identity frames to a special Layer 2 address (01:80:C2:00:00:03) on the local network segment. The supplicant listens on this address, and on receipt of the EAP-Request Identity frame it responds with an EAP-Response Identity frame containing an identifier for the supplicant such as a User ID. The authenticator then encapsulates this Identity response in a RADIUS Access-Request packet and forwards it on to the authentication server. The supplicant may also initiate or restart authentication by sending an EAPOL-Start frame to the authenticator, which will then reply with an EAP-Request Identity frame.

Negotiation(Technically EAP negotiation) The authentication server sends a reply (encapsulated in a RADIUS Access-Challenge packet) to the authenticator, containing an EAP Request specifying the EAP Method (The type of EAP based authentication it wishes the supplicant to perform). The authenticator encapsulates the EAP Request in an EAPOL frame and transmits it to the supplicant. At this point the supplicant can start using the requested EAP Method, or do an NAK ("Negative Acknowledgement") and respond with the EAP Methods it is willing to perform.

Authentication If the authentication server and supplicant agree on an EAP Method, EAP Requests and Responses are sent between the supplicant and the authentication server (translated by the authenticator) until the authentication server responds with either an EAP-Success message (encapsulated in a RADIUS Access-Accept packet), or an EAP-Failure message (encapsulated in a RADIUS Access-Reject packet). If authentication is successful, the authenticator sets the port to the "authorized" state and normal traffic is allowed, if it is unsuccessful the port remains in the "unauthorized" state. When the supplicant logs off, it sends an EAPOL-logoff message to the authenticator, the authenticator then sets the port to the "unauthorized" state, once again blocking all non-EAP traffic.

An open source project known as Open1X produces a client, Xsupplicant. This client is currently available for both Linux and Windows. The main drawbacks of the Open1X client are that it does not provide comprehensible and extensive user documentation and the fact that most Linux vendors do not provide a package for it. The more general wpa_supplicant can be used for 802.11 wireless networks and wired networks. Both support a very wide range of EAP types.[8]

Windows defaults to not responding to 802.1X authentication requests for 20 minutes after a failed authentication. This can cause significant disruption to clients.

The block period can be configured using the BlockTime value in the registry. A hotfix is required for Windows XP SP3 and Windows Vista SP2 to make the period configurable.[12]

Wildcard server certificates are not supported by EAPHost, the Windows component that provides EAP support in the operating system.[13] The implication of this is that when using a commercial certification authority, individual certificates must be purchased.

Windows XP has major issues with its handling of IP address changes that result from user-based 802.1X authentication that changes the VLAN and thus subnet of clients.[14] Microsoft has stated that it will not back port the SSO feature from Vista that resolves these issues.[15]

If users are not logging in with roaming profiles, a hotfix must be downloaded and installed if authenticating via PEAP with PEAP-MSCHAPv2.[16]

Windows Vista based computers that are connected via an IP phone may not authenticate as expected and, as a result, the client can be placed into the wrong VLAN. A hotfix is available to correct this.[17]

For most enterprises deploying and rolling out operating systems remotely, it is worth noting that Windows PE does not natively have any support for 802.1X. However, support can be added to WinPE 2.1[19] and WinPE 3.0[20] through hotfixes that are available from Microsoft. Although full documentation is not yet available, preliminary documentation for the use of these hotfixes is available via a Microsoft blog.[21]

Not all devices support 802.1X authentication. Examples include network printers, Ethernet-based electronics like environmental sensors, cameras, and wireless phones. For those devices to be used in a protected network environment, alternative mechanisms must be provided to authenticate them.

One option would be to disable 802.1X on that port, but that leaves that port unprotected and open for abuse. Another, slightly more reliable option is to use the MAB option. When MAB is configured on a port, that port will first try to check if the connected device is 802.1X compliant, and if no reaction is received from the connected device, it will try to authenticate with the AAA server using the connected device's MAC address as username and password. The network administrator then must make provisions on the RADIUS server to authenticate those MAC-addresses, either by adding them as regular users, or implementing additional logic to resolve them in a network inventory database.

In the summer of 2005, Microsoft's Steve Riley posted an article detailing a serious vulnerability in the 802.1X protocol, involving a man in the middle attack. In summary, the flaw stems from the fact that 802.1X authenticates only at the beginning of the connection, but after that authentication, it's possible for an attacker to use the authenticated port if he has the ability to physically insert himself (perhaps using a workgroup hub) between the authenticated computer and the port. Riley suggests that for wired networks the use of IPsec or a combination of IPsec and 802.1X would be more secure.[28]

EAPOL-Logoff frames transmitted by the 802.1X supplicant are sent in the clear and contain no data derived from the credential exchange that initially authenticated the client.[29] They are therefore trivially easy to spoof on shared media, and can be used as part of a targeted DoS on both wired and wireless LANs. In an EAPOL-Logoff attack a malicious third party with access to the medium the authenticator is attached to, repeatedly sends forged EAPOL-Logoff frames from the target device's MAC Address. The authenticator (believing that the targeted device wishes to end its authentication session) closes the target's authentication session, blocking traffic ingressing from the target, denying it access to the network.

The 802.1X-2010 specification, which began as 802.1af, addresses vulnerabilities in previous 802.1X specifications, by using MACSec IEEE 802.1AE to encrypt data between logical ports (running on top of a physical port) and IEEE 802.1AR (Secure Device Identity / DevID) authenticated devices.[5][6][30][31]

As a stopgap until these enhancements are widely implemented, some vendors have extended the 802.1X-2001 and 802.1X-2004 protocol, allowing multiple concurrent authentication sessions to occur on a single port. While this prevents traffic from devices with unauthenticated MAC addresses ingressing on an 802.1X authenticated port, it will not stop a malicious device snooping on traffic from an authenticated device and provides no protection against MAC spoofing, or EAPOL-Logoff attacks.